The present invention relates to a borehole data transmission apparatus and more particularly to a borehole data transmission apparatus utilizing a transducer longitudinally expansive in response to magnetic field pulses for generating pressure pulses in a borehole through drilling mud to the surface of the earth.
It is well known to transmit measurement data from a measuring instrument at the downhole end of a drill string within a borehole by generating pressure variations within the mud flow passing along the drill string and retrieving the transmitted data by sensing such pressure variations at the surface. Typically such data includes weight on the bit, RPM, natural gamma, formation resistivity, bottom hole temperature, bottom hole pressure, and virtually any information related to detectable conditions. Such information is usually transmitted from the bottom of the borehole to the surface as a series of hydraulic pulses produced in, and transmitted through, the drilling fluid. At the surface, the pressure signals are detected, usually converted to electrical signals, and processed to provide information in a useable format.
It is common to create pressure pulses in drilling fluid by periodically restricting the passage of drilling fluid flowing through the drill string, or periodically diverting a portion of the flow into the annulus of the drill string, to form a series of pulses in the drilling fluid which is normally pressurized and which normally flows down through the drill string and back up the annulus around the outer surface of the drill string.
Thus, for example, a variety of systems have been used to form the pressure pulses and to effect transmission to the surface. In most instances, the pulsing system involves some form of valve so positioned that the entire fluid flow through the drill string also flows through the valve.
It is known to use rotary "turbine-like" valves which include a rotor and a stator. Both the rotor and stator have slots which can be aligned to completely open the valve and let drilling fluid pass through without any obstruction, or can be misaligned to partially close the valve and provide a strong resistance to the passing of the drilling fluid. Turbines have also been used for generating the electrical power needed to operate the logging-while-drilling systems.
The recognized difficulty with conventional techniques for generating pressure pulses using valves and turbine-like valves is the limited service life of the apparatus caused by the drilling fluid. The drilling fluid, due to its composition and the rate of flow, tends to be quite abrasive. As a result of the abrasive conditions of the drilling fluid and the fact that the transmission of data may involve a significant number of pulses, the life of the valves is limited. Additionally, conventional downhole data transmission devices utilizing valves and turbine-like valves must have a relatively complex design and at least two parts (i.e. puppet valve and seat, or rotor and stator) working in the drilling fluid in order to generate the required pressure pulses. Still another difficulty with conventional telemetry systems is the poor quality of the pressure signals received at the surface due to the high noise level inside the drilling fluid column.
It is an object of the present invention to provide an autonomous, simple, inexpensive borehole data transmission device with sensitive components that are completely isolated from the abrasive flow of drilling mud thereby prolonging the service life of the borehole data transmission device.
It is another object of the present invention to provide a borehole data transmission device having a high data transmission rate.
It is still a further object of this invention to provide a borehole data transmission device capable of providing improved signal reception at the surface.
It is still another object of the present invention to provide a borehole data transmission device that does not require the flow of drilling fluid to generate pressure pulse signals.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.